Generally, stereophonic signals include a left channel input signal and a right channel input signal. A sum signal is obtained by adding the two signals whereas a difference signal is obtained by subtracting one signal from the other.
It is known to use sound retrieval systems (SRS) to retrieve sound more closely resembling an original sound, to generate three dimensional sound images using two speakers and to expand the audible area regardless of input signals of either mono, stereo or encoded surround sound. According to the fundamental principle of SRS, a three dimensional signal and directional cues of an audio system are provided through the process of treating direct sound and centralized sound such as dialogue, vocalist and soloist, from the sum signal (L+R), and ambiance signals such as reflective sound and reverberation.
In other words, SRS is a sound treatment technique based on the human hearing system and may be distinguished from a conventional stereo system or a sound expansion technique. Therefore, SRS may not need such operations as time delay, phase shift, and encoding or decoding.
Another characteristic feature of conventional SRS is that it is generally not affected by the position of speakers, thereby enabling three dimensional stereo sound, similar to a live performance, regardless of a listener's position. When a stereo microphone is used for recording, it may be difficult for a certain frequency such as that of side sound to be properly retrieved because the microphone does not respond to the frequency in the same way as human ears. However, the SRS can reproduce the frequency and the ratio of direct sound and indirect sound so that a listener can hear sounds quite close to the original.
As shown in FIG. 1, an SRS generally includes stereo image enhancement means 10 and perspective correction means 30. Each of these means can also be used as an independent SRS. The stereo image enhancement means 10 receives a left input sound signal L.sub.in and a right input sound signal R.sub.in and, after selective enhancement, outputs a first left signal L.sub.out1 and a first right signal R.sub.out1. The perspective correction means 30 receives the output signals L.sub.out1 and R.sub.out1 from the stereo image enhancement means 10 and, after correcting the signals toward the direction of sound source regardless of the position of the speakers, outputs a second left signal L.sub.out2 and a second right signal R.sub.out2.
Thus, as shown in FIG. 1, a stereophonic device using conventional SRS comprises stereo image enhancement means 10 for outputting first audio signals to the left L.sub.out1 and to the right R.sub.out1 after first receiving audio input signals from the left L.sub.in and from the right R.sub.in, then enhancing a difference signal of the two input signals. The stereophonic device also comprises perspective correction means 30 for outputting second audio signals to the left L.sub.out2 and to the right R.sub.out2 after receiving the first audio signals L.sub.out1 and R.sub.out1 from the stereo image enhancement means 10, then correcting the signals toward the direction of sound source regardless of the position of the speakers.
In the stereo image enhancement means 10, as shown in FIG. 2, a first high-pass filter 11 receives a left input sound signal L.sub.in and a second high-pass filter 12 receives the right input sound signal R.sub.in. Both input signals are filtered through 30 kHz high-pass filters 11 and 12 so that the audio system can be protected from excessive low frequency energy which may occur due to a physical impact.
A first adder 13 receives and adds the output signals from the first high-pass filter 11 and the second high-pass filter 12, generating a sum signal (L+R). A first subtracter 14 receives the output signals from the first high-pass filter 11 and the second high-pass filter 12, generating a difference signal (L-R). In such a manner, the sum signal (L+R) or the difference signal (L-R) is formed from the two input signals after passing through the high-pass filters 11 and 12.
The difference signal (L-R) is input to a spectrum analyzer 15 which includes, for example, seven band-pass filters. The spectrum analyzer 15 classifies the frequency of the difference signal (L-R) into 7-bands and outputs them.
The dynamic sum signal equalizer 17, after receiving the sum signal (L+R) and the output signal from the spectrum analyzer 15, outputs a sum signal (L+R).sub.p which is equalized by the equalizing control signal X1. The dynamic difference signal equalizer 18, after receiving the difference signal (L-R) and the output signal from the spectrum analyzer 15, outputs a difference signal (L-R).sub.p which is equalized by the equalizing control signal X1.
Each of the 7-band output signals from the spectrum analyzer 15, after passing through an internal rectifying circuit and buffer, is input to a dynamic sum signal equalizer 17 and to a dynamic difference signal equalizer 18 as a control signal. Each of the dynamic equalizers 17 and 18 also includes seven band-pass filters which are characterized by the output signal from the spectrum analyzer 15.
The band-pass filters accentuate a low-frequency component in comparison to a high-frequency component. As a result, a signal of the dynamic difference equalizer 18 at same band frequency is attenuated according to the scale of output signal from the band-pass filter of the spectrum analyzer 15. For the sum signal (L+R), a large component of the difference signal (L-R) may be amplified more than a small component, resulting in an increase of the difference between the large component and the small component to effect enhancement of stereo image through successive processes thereafter. Each of the band-pass filters of the spectrum analyzer 15 and of the dynamic equalizers 17 and 18 preferably includes seven intervals per octave. Frequencies in the middle of the intervals are 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz and 8 kHz.
A fixed equalizer 19 receives the difference signal (L-R).sub.p from the dynamic difference signal equalizer 18 and outputs an attenuated signal in the band from 1 kHz to 4 kHz. Inadequate accentuation of the signals may be prevented at the frequency band from 1 kHz to 4 kHz which is a sensitive region to human ears.
A control circuit 16 receives the sum signal (L+R) from the first adder 13, the difference signal (L-R) from the first subtracter 14 and the feedback control signal X3, and then controls the sum signal (L+R) and the processed difference signal (L-R).sub.p to a certain ratio. Thus, artificial reverberation may be prevented from erroneously boosting and outputting an equalizing control signal X1 and multiplying control signal X2.
In other words, if artificial reverberation is regarded as a small difference signal (L-R), the signal at the same band may be amplified to generate unpleasant sound. When the scale of the processed difference signal (L-R).sub.p exceeds a predetermined ratio even though the sum signal (L+R) is large enough, the difference signal may be regarded as an artificial reverberation and may be controlled continuously. Such control may be carried out restrictively for the frequency band of 500 Hz, 1 kHz and 2 kHz where the frequency of a soloist or vocalist predominates.
A first multiplier 21 multiplies the output signal from the dynamic sum signal equalizer 17 and a first correction factor K1 and outputs the resulting signal. A second multiplier 22 multiplies the output signal from the fixed equalizer 19 and a multiplying control signal X2 and outputs a feedback control signal X3. A third multiplier 23 multiplies the output signal from the second multiplier 22 and a second correction factor K2 and outputs the resulting signal. After the above described operations, the audio signal is further treated by the first correction factor K1 and the second correction factor K2, resulting in a final stereo image enhancement signal.
The operations performed by the stereo image enhancement means 10 as described above can thus be expressed by the following equations: EQU L.sub.out1 =L.sub.in +K1 (L+R).sub.p +K2 (L-R).sub.p ( 1) EQU R.sub.out1 =R.sub.in +K1 (L+R).sub.p +K2 (L-R).sub.p ( 2)
In equations (1) and (2), one of the main characteristics of the stereo image enhancement means 10 is that relatively small component of the difference signal (L-R) may be amplified selectively.
A fourth multiplier 24 multiplies the output signal from the third multiplier 23 and -1. A second adder 25 adds the output signals from the first high-pass filter 11, from the first multiplier 21 and from the third multiplier 23 and outputs the resulting left output signal L.sub.out1. A third adder 26 adds the output signals from the second high-pass filter 12, from the fourth multiplier 24 and from the first multiplier 21 and outputs the resulting right output signal R.sub.out1.
Thus, as shown in FIG. 2, the stereo image enhancement means 10 comprises: a first high-pass filter 11 for outputting a signal after filtering the input signal L.sub.in ; a second high-pass filter 12 for outputting a signal after filtering the input signal R.sub.in ; a first adder 13 for outputting a sum signal (L+R) after adding both of the output signals from the first high-pass filter 11 and the second high-pass filter 12; and a first subtracter 14 for outputting a difference signal (L-R) after subtracting the output signal of the second high-pass filter 12 from the output signal of the first high-pass filter 11. The stereo image enhancement means 10 also comprises a spectrum analyzer 15 for outputting signals after classifying the frequency of difference signal (L-R) into 7-band; a dynamic sum signal equalizer 17 for outputting a sum signal (L+R).sub.p after receiving the sum signal (L+R) from the adder 13 and an output signal from the spectrum analyzer 15 which are equalized by an equalizing control signal X1; a dynamic difference signal equalizer 18 for outputting a difference signal (L-R).sub.p after receiving the difference signal (L-R) from the subtracter 14 and the output signal from the spectrum analyzer 15 which are equalized by the equalizing control signal X1; and a fixed equalizer 19 for receiving the difference signal (L-R).sub.p from the dynamic difference signal equalizer 18 and attenuating the frequency of the signal in the band from 1 kHz to 4 kHz before outputting the signal.
The stereo image enhancement means 10 also comprises a control circuit 16 for outputting the equalizing control signal X1 and a multiplying control signal X2 after receiving the sum signal (L+R) from the first adder 13, the difference signal (L-R) from the first subtracter 14 and a feedback control signal X3, and then controlling the sum signal (L+R) and the difference signal (L-R) to a certain ratio and preventing artificial reverberation from erroneous boosting; a first multiplier 21 for multiplying a first correction factor K1 and an output signal from the dynamic sum signal equalizer 17; a second multiplier 22 for generating the feedback control signal X3 after multiplying the output from the fixed equalizer 19 and the control signal X2; a third multiplier 23 for multiplying the output from the second multiplier 22 and a second correction factor K2; and a fourth multiplier 24 for multiplying the output from the third multiplier 23 and -1.
The stereo image enhancement means 10 also comprises a second adder 25 for outputting a left signal L.sub.out1 after adding the output from the first high-pass filter 11, the output from the first multiplier 21 and the output from the third multiplier 23; and a third adder 26 for outputting a right signal R.sub.out1 after adding the output from the second high-pass filter 12, the output from the fourth multiplier 24 and the output from the first multiplier 21.
The perspective correction means 30 of FIG. 1 will now be described. When a speaker is positioned in the front or at the side like the door speakers of a car, or when a headphone is used, the perspective of side component of sound or central component of sound may be corrected by the perspective correction means.
FIGS. 3A to 3D are curves showing the frequency characteristics corresponding to the positions of a sound source. FIG. 3A shows a curve of the frequency perceived by human ears when the sound source is in the front, and FIG. 3B shows a curve of the frequency when the sound source is at a right angle. As shown, the same level of sound may be perceived differently by human ears according to the position of sound source and the frequency.
FIG. 3C shows a curve of the frequency when the sound source is in the front while the speaker is positioned at the side. For example, when a headphone is used, an equalizer may be necessary for correcting the direction of central sound component or front sound component. FIG. 3D shows, similarly, that an equalizer may be necessary for correcting the side sound component from the front positioned speaker.
Referring to FIG. 4, the performance of perspective correction means 30 will now be described. As shown in FIG. 4, the perspective correction means 30 comprises: a first adder 31 for generating a sum signal (L+R) after adding the left input signal L.sub.in or L.sub.out1 and the right input signal R.sub.in or R.sub.out1 ; a first subtracter 32 for generating a difference signal (L-R) after subtracting the right input signal R.sub.in from the left input signal L.sub.in ; a fixed sum signal equalizer 33 for generating a sum signal (L+R).sub.s after equalizing the sum signal (L+R); and a fixed difference signal equalizer 34 for generating a difference signal after equalizing the difference signal (L-R).sub.s.
The perspective correction means 30 also includes a first selecting means 35 for selecting either the sum signal (L+R) or the equalized sum signal (L+R).sub.s in response to a selecting signal S; a second selecting means 36 for selecting either the difference signal (L-R) or the equalized difference signal (L-R).sub.s in response the selecting signal S; and a first multiplier 37 for multiplying an output signal from the second selecting means 36 and -1. The perspective correction means 30 also includes a second adder 38 for generating a second left output signal L.sub.out2 after adding output signals from the first selecting means 35 and from the second selecting means 36; and a third adder 39 for generating a second right output signal R.sub.out2 after adding output signals from the first selecting means 35 and from the first multiplier 37.
The first adder 31 outputs the sum signal (L+R) after adding the left input signal L.sub.in or L.sub.out1 and the right input signal R.sub.in or R.sub.out1. The first subtracter 32 outputs the difference signal (L-R) after subtracting the right input signal R.sub.in from the left input signal L.sub.in. Thus, the sum signal (L+R) or the difference signal (L-R) is generated from the left input signal and the right input signal, which is input to the fixed sum signal equalizer 33 and the fixed difference signal equalizer 34 respectively.
The fixed sum signal equalizer 33 outputs a processed sum signal (L+R).sub.s after equalizing the inputted sum signal (L+R). The fixed difference signal equalizer 34 outputs a processed difference signal (L-R).sub.s after equalizing the inputted difference signal (L-R). The characteristic of the fixed sum signal equalizer 33, as shown in FIG. 3C, is that a correction configuration is generally required to compensate the central sound component from the side speaker, whereas the fixed difference signal equalizer 34, as shown in FIG. 3D, generally requires a correction configuration to compensate the side sound component from the front positioned speaker.
The first selecting means 35 is a multiplexer for selecting one of the two input signals, the sum signal (L+R) and the processed sum signal (L+R).sub.s, in response to the selecting signal S. The second selecting means 36 selects either the difference signal (L-R) or the processed difference signal (L-R).sub.s in response to the selecting signal S.
The first multiplier 37 multiplies the output signal from the second selecting means 36 and -1, outputting the resultant signal. The second adder 38 outputs the second left output signal L.sub.out2 after adding the output signals from the first selecting means 35 and from the second selecting means 36. The third adder 39 outputs the second right output signal R.sub.out2 after adding the output signals from the first selecting means 35 and from the first multiplier 37.
Thus, the final output signals, i.e. the second left output signal L.sub.out2 and the second right output signal R.sub.out2, are generated through a mixing circuit of the second adder 38 and the third adder 39. The above described process may be expressed by the following equations: EQU L.sub.out =(L+R).sub.s +(L-R).sub.s ( 3) EQU R.sub.out =(L+R).sub.s -(L-R).sub.s ( 4)
where (L+R).sub.s and (L-R).sub.s respectively represent the sum signal and the difference signal which are processed in the equalizer in response to the selecting signal S.
According to equations (3) and (4), when the selecting signal S selects the first terminal of the first selecting means 35 or the second selecting means 36, the system is configured for compensating the side sound signal from the front speaker, wherein the difference signal (L-R).sub.s is compensated as shown in FIG. 3D whereas the sum signal (L+R).sub.s remains untreated because the speaker is in the front. Conversely, when the selecting signal S selects the second terminal of the first selecting means 35 or the second selecting means 36, the system is configured for compensating the front sound signal from the side speaker.
In such an instance, the characteristic of the fixed sum signal equalizer 33 and the fixed difference signal equalizer 34 need not be as accurate as shown in FIG. 3C or 3D. It may be sufficient to equalize only those main frequencies, such as 500 Hz, 1 kHz and 8 kHz, the characteristics of which are listed in the following Table.
TABLE ______________________________________ DIFF. SIGNAL SUM SIGNAL MAIN FREQUENCY EQUALIZER EQUALIZER ______________________________________ 500 Hz +5.0 dB -5.0 dB 1 kHz +7.7 dB -7.5 dB 8 kHz +15.0 dB -15.0 dB ______________________________________
In conclusion, the SRS, regardless of the recorded sound source, is capable of retrieving the original stereo image, extending the scope of hearing and recovering the directional cues of the original sound source. In addition, the SRS may be advantageous compared with other sound control systems such as Dolby Prologic which may restrict the sound source or other effect processors which may require additional delay.